The effector, or Fc, regions of antibodies bind to various receptors on many different cell types. One such receptor is the CD32a IgG receptor (also known as FcgammaRIIa). It has been reported that human platelets and other human cells, such as basophils, eosinophils, monocytes, neutrophils, dendritic cells, macrophages, and mast cells, display cell surface CD32a receptors (Hogarth P M et al. Fc receptor-targeted therapies for the treatment of inflammation, cancer and beyond (March 2012) Nat Rev Drug Discov 11:311; PubMed ID: 22460124; Bruhns P. Properties of mouse and human IgG receptors and their contribution to disease models (June 2012) Blood 119:5640; PubMed ID: 22535666). Activation of CD32a by Fc regions of IgG antibodies (regardless of antigen specificity) results in a number of in vivo reactions, many of which have negative consequences for the human host. For example, IgG activation of CD32a can contribute to fatality in heparin-induced thrombocytopenia (HIT; see Boon D M et al. Heparin-induced thrombocytopenia and thrombosis: a potential fatal complication in a routine treatment (March 1995) Neth J Med 46:146; PubMed ID: 7731489; and Warkentin T E et al. Sera from patients with heparin-induced thrombocytopenia generate platelet-derived microparticles with procoagulant activity: an explanation for the thrombotic complications of heparin-induced thrombocytopenia (December 1994) Blood 84:3691; PubMed ID: 7949124). It has also been reported that IgG-mediated activation of CD32a on neutrophils, monocytes, and macrophages promotes airway inflammation, allergic reactions, and anaphylaxis. See, e.g. Jönsson F. et al. Human Fc-gamma-RIIA induces anaphylactic and allergic reactions (2012 March 15) Blood 119:2533-44, PubMed ID: 22138510. Activation of CD32a by IgG-Fc can also contribute to thrombosis in HIT (see, e.g. Arepally G et al. Fc gamma RIIA H/R 131 polymorphism, subclass-specific IgG anti-heparin/platelet factor 4 antibodies and clinical course in patients with heparin-induced thrombocytopenia and thrombosis (January 1997) Blood 89:370; PubMed ID: 9002937; Newman P M et al. Heparin-induced thrombocytopenia: new evidence for the dynamic binding of purified anti-PF4-heparin antibodies to platelets and the resultant platelet activation (July 2000) Blood 96:182; PubMed ID: 10891449; Jaffray B et al. Fatal venous thrombosis after heparin therapy (March 1991) Lancet 337:561; PubMed ID: 1671929).
In a 2012 report by Jönsson et al., the authors reported that blocking the CD32a receptor protected mice from local and systemic anaphylaxis, and concluded that “[t]argeting Fc[gamma]RIIA with specific blocking molecules in inflammation and autoimmune/allergic reactions in humans might lead to similar inhibition as we reported recently for mouse Fc[gamma]RIIIA in a murine model of rheumatoid arthritis.” Id. at 2542. Jönsson continued that “[b]locking Fc[gamma]RIIA using divalent ligands (eg, mAb IV.3) to prevent allergic and autoimmune disease in humans, however, should not be envisioned, as we report here that high-doses of mAb IV.3 induced rather than prevented anaphylaxis.” Id. at 2542 (emphasis added). Thus, while blockade of CD32a was a desired goal for treating inflammatory, autoimmune and allergic disorders, those of skill in the art did not envision blockade with CD32a antibodies due to their known negative side effects upon in vivo administration. The inventors have now solved this problem by providing novel CD32a antibodies that do not elicit negative side effects such as anaphylaxis.
In addition to diseases and disorders mediated by activation of CD32a, a number of diseases and disorders are mediated by CD32a interactions with the Fc regions of immobilized IgG, which do not directly activate CD32a. “Immobilized IgG” refers to antibody molecules that are bound to, or precipitated on, a surface and thus have restricted mobility (i.e., are “immobilized”). Cells having immobilized IgG may alternatively be described as “IgG-coated” cells. CD32a is known to interact only weakly with the Fc region of single IgG molecules, whether soluble (Hogarth P M et al. Fc receptor-targeted therapies for the treatment of inflammation, cancer and beyond (March 2012) Nat Rev Drug Discov 11:311; PubMed ID: 22460124) or immobilized (Wines B D et al. The IgG Fc contains distinct Fc receptor (FcR) binding sites: the leukocyte receptors Fc gamma RI and Fc gamma RIIa bind to a region in the Fc distinct from that recognized by neonatal FcR and protein A (May 2000) J Immunol 164:5313; PubMed ID: 10799893). Thus, antibodies incapable of directly activating CD32a nevertheless caused CD32a-mediated diseases and disorders such as thrombocytopenia when such antibodies were immobilized on the platelet surface (McKenzie et al. The role of the human Fc receptor FcgammaRIIA in the immune clearance of platelets: a transgenic mouse model (April 1999) J Immunol 162:4311; PubMed ID: 10201963).
IgG-coated platelets (or other cells) are actively cleared from the circulating blood. For example, it is well known that in immune thrombocytopenic purpura (ITP), human patients with circulating anti-platelet antibodies (typically IgG) experience platelet clearance mediated in large part by the spleen and the liver, where Fc-receptors (including CD32a) on phagocytes bind and retain the IgG-coated platelets. Removal of the spleen (splenectomy) can alleviate this condition. Unlike with HIT, however, thrombosis is not typically associated with the clearance of IgG-coated platelets in ITP; rather, the clinical problem of bleeding is the more prominent concern, and improved therapeutic strategies for this problem are needed (Altomare I et al. Bleeding and mortality outcomes in ITP clinical trials: a review of thrombopoietin mimetics data (October 2012) Am J Hematol 87:984; PubMed ID: 22729832).
CD32a is also known to mediate clearance of IgG-coated red blood cells (erythrocytes) in CD32a mediated diseases and disorders such as autoimmune hemolytic anemia (AIHA). Targeting CD32a with blocking mAbs would thus seem to be of great utility in treating AIHA; indeed, this was reported with the anti-CD32 mAb, MDE-8, which was shown to ameliorate IgG antibody-induced anemia in mice having a human CD32a transgene but otherwise lacking classical mouse IgG receptor function—that is, the animals used to test MDE-8 lacked functional mouse IgG receptors of type I (CD64) and type III (CD16), leaving open the question as to how these might affect MDE-8 activity in vivo (van Royen-Kerkhof A et al. A novel human CD32 mAb blocks experimental immune haemolytic anaemia in FcgammaRIIA transgenic mice (July 2005) Br J Haematol 130:130; PubMed ID: 15982355). MDE-8 has not been developed as a therapeutic antibody. Reasons for the lack of preclinical development of MDE-8 have not been publicly disclosed. However, the inventors have now identified and solved a previously undescribed problem with MDE-8 and other anti-CD32a antibodies, namely by modifying them to reduce binding to IgG Fc-receptors, and so that they no longer mediate clearance via CD32a when immobilized on cells, thereby making clinical development possible.
Compositions that can prevent CD32a-mediated clearance of IgG-coated cells without causing negative side effects are therefore desired. The inventors herein describe such compositions and detail their successful use to treat and prevent CD32a-mediated diseases and disorders.